1. Field of the Invention
This invention relates to oil separation systems for use with refrigeration systems and more particularly to an apparatus for returning oil, separated from a mixture of refrigerant and lubricating oil, to the compressor of a refrigeration system.
2. Description of the Prior Art
Refrigeration systems, of the type used in transport refrigeration equipment, include, in the simplest form, a compressor, a condenser, an expansion device and an evaporator serially interconnected to form a closed refrigeration circulation path. Typically, such systems use a reciprocating type compressor which is lubricated by a lubricating oil which mixes with the refrigerant being compressed therein. In such transport refrigeration systems, the refrigerant R-12 has been used almost exclusively with satisfactory results. Since R-12 is completely miscible with the lubricating oils used in such systems, no problem has been experienced with the return of the oil to the compressor crank case during the wide range of operating conditions which a transport refrigeration system experiences. Also, since the oil and refrigerant remain mixed, even at low system temperatures, the thermodynamic characteristics of systems using completely miscible refrigerants are predictable. As a result, it has not been necessary to utilize oil separators in such systems. R-12, and many other completely miscible refrigerants, however, are in the family of refrigerants known as chlorofluorocarbons (CFCs). For many years CFCs have seen wide spread use in many different refrigeration applications. The reason for such popularity is that CFCs are stable and non-flammable, boil and condense in a useful temperature and pressure range, and were believed relatively inert and free of harmful side effects. Recently, there has been a growing scientific consensus that emissions of CFCs are depleting a layer of stratospheric ozone that protects the earths surface from the harmful effects of ultraviolet radiation. In response to this concern, international agreements have been entered into which require the parties thereto to reduce significantly their production and consumption of certain ozone--depleting CFCs and halons over the next decade. Further, federal and state bills, that seek to regulate use, manufacture, importation, and disposal of CFCs, are being considered.
A suitable substitute refrigerant for R-12 in low temperature applications, is R-22. R-22 is a member of a chemical family known as hydroclorofluorocarbons (HCFCs). It is believed that because of their hydrogen component, the HCFCs break down substantially in the lower atmosphere and their ozone depletion potential is substantially lower than that of R-12 and other CFC refrigerants.
R-22 is only partially miscible with lubricating oils. Below a certain temperature a partially miscible refrigerant-oil system may separate into two phases. One solution being particularly rich in oil and the other refrigerant rich. Such separation can result in detrimental heat transfer characteristics, as well as problems wherein the oil rich layer separates and collects in pockets or blind passages and thus the oil is not returned to the compressor sump. To alleviate this problem oil separators are frequently required in the discharge line to minimize oil circulation when partially miscible refrigerants are used in systems, particularly those involving low evaporator temperatures.
In such systems it is desirable to extract or separate the entrained oil from the refrigerant as quickly as possible after compression, to minimize the effects of the oil on other parts of the system. Furthermore, it is desirable to remove the entrained oil while the refrigerant is in the gaseous state since less separating effort is required, the refrigerant is easier to handle, and differential pressures are present in the gaseous state portion of the system for aiding the separation process. It is also desirable to provide a means for returning the separated oil back to the compressor sump.
A typical prior art oil separator comprises a housing having an inlet for the refrigerant/oil mixture, an outlet for the substantially oil free refrigerant gas and a return means for conducting the separated oil from the separator back to the compressor. Typically, a strainer is arranged about the inlet for the refrigerant/oil mixture and about the outlet for the refrigerant gas. The strainers are configured such that the one on the inlet catches oil on the inside and the one on the outlet catches oil on the outside. The strainers are typically separated by a baffle and allow separated oil to drip into the lower portion of the housing. In the lower portion, close to the bottom of the housing, a float assembly is positioned. The float assembly typically comprises a float ball attached to a lever arm for operating a needle valve within a valve body to open and close a valve seat and allow oil to be sent through a return line to the compressor oil sump. U.S. patents showing oil separating systems typical of that described hereinabove include U.S. Pat. No. 3,283,532 --Refrigerating Apparatus with Oil Separating Means to Eric J. Kocher and U.S. Pat. No. 4,310,338 --Replaceable Float Oil Separator to Earnest W. Shoemacher, et. al.
The oil separators of the type described above having mechanical float valves are not suitable for transport refrigeration applications because of the shock and vibration which such systems experience when they are on the road.
Other known types of oil return arrangements include the use of an electric solenoid in an oil return line. Such systems are not deemed suitable for a transport refrigeration application primarily because of the large number of operating cycles expected, coupled with, the requirement that such systems be extremely reliable, and capable of providing many years of trouble free operation.
U.S. Pat. No. 3,070,977 --Refrigeration System including Oil Separator and Muffler Unit and Oil Return Arrangement to C.J. Kimmel, et al. discloses the use of a standard orifice in an oil return line wherein the orifice is configured so that it passes oil freely, but restricts the passage of gas therethrough. The theory of the '977 patent is that when no oil is present in the region communicating with the entrance to the orifice and that region is occupied by a refrigerant gas, the difference in pressure across the orifice creates a "pressure shock" or "baffle shock" condition, so that no significant amount of gas flows through the orifice. In other words, the orifice itself acts as an open valve for the oil but acts as a substantially closed valve for the gas. Kimmel also employs a mechanical needle valve arrangement to control flow from the separator.
A capillary tube is another device which is selectively more receptive to the flow of a liquid than a vapor. Basically, liquid passes through a capillary much easier than a vapor, because the pressure drop experienced by vapor causes a decrease in density which in turn causes an increase in velocity to the point where sonic velocity (or "choked flow") is reached.
The use of a capillary in an oil return line of an oil separator is shown in U.S. Pat. No. 4,800,736 to Weber.
Transport refrigeration systems are subject to an extremely wide range of operating conditions and must be capable of operating reliably and maintaining a desired setpoint temperature over that wide range of conditions. As an example, a transport refrigeration system of the self contained, independent engine-driven type used for cooling trailers may be required to maintain a range of cargo temperatures from 55.degree. F., for bananas, to -20.degree. F. for a cargo of ice cream. The system further must be designed to have the capability of maintaining these temperatures when the ambient temperature ranges from -20.degree. F. to 120.degree. F.
It should be appreciated that during certain operating conditions of a transport refrigeration system, for example, during the initial pull down of the temperature of cargo which has been loaded at a temperature far greater than the desired setpoint, the amount of oil collected within the sump of the oil separator may vary and it is desirable to have an oil return system capable of providing variable rates of oil return from the separator to the compressor sump. It should also be appreciated that during the initial startup and operation of such systems, at the low range of the ambient temperatures which such a system may see, the viscosity of the lubricating oil will be such that return of oil to the compressor sump would be extremely difficult.